Why is 1 butanol sn2?

Well, it isn't really the case that 1-butanol "is SN2", but it does preferentially undergo SN2 reactions.

Being a primary alcohol, if 1-butanol was to undergo an SN1 reaction, this would require the formation of an intermediate, which would need to be a primary carbocation. These are not stable species (although tertiary carbocations are). Also, approach to the hydroxy group is not hindered. Therefore, SN2 is far more favourable.